Issue link: https://iconnect007.uberflip.com/i/241303
article an introduction to rigid-flex design best practices continues Rules for the project aim for successful production and eliminating preventable re-spins. Golden Rules • Communicate with the fabricator! • Qualify the fabricator's capability to build the planned rigid-flex design. • Involve the fabricator as early as possible in the design process. • Collaborate so the design's layer stack matches the fabricator's processes. • Use IPC-2223 as the common point of reference with the fabricator. Otherwise, communication in the form of documentation can cause errors and misunderstandings resulting in costly delays. • Graduate from delivering Gerber files to the fabricator. Instead, deliver files in ODB++ (v7.0 or later) or in a format that meets IPC2581 because either format identifies specific layer types for clear documentation. For a successful rigid-flex design, the design team must also select the materials that balance, as always, cost versus performance. Most conventional PCB boards start with a rigid fiberglass/epoxy substrate. Although termed "rigid," fiberglass/epoxy substrates exhibit some flexibility, but not enough for more complex applications that involve movement. For 3D rigid-flex designs, dimensionally stable, flexible and heat-resistant polyimide (PI) film is the most common choice. It remains reasonably stable due to low thermal expansion and contraction (relative to PET) while also tolerating multiple reflow cycles. Polyester (PET), also commonly used, but does not tolerate high temperatures well and is less dimensionally stable than PI. As well, thin fiberglass/epoxy cores also find application in rigid-flex circuits. Besides substrates, the design will require additional films (usually PI or PET, but sometimes flexible solder mask ink) for coverlay. The coverlay protects the outer surface components and conductors from damage and corrosion and insulates the conductors as well. By definition, rigid-flex circuits impose additional requirements when selecting conductor materials. Electrodeposited (ED) copper foils used in traditional PCB designs fall short of the necessary flexibility and toughness properties needed in a rigid-flex design. Rigid-flex designs utilize a variety of higher-performance conductor materials and methods. However, the two most common are medium-priced high-ductility electrodeposited (HD-ED) and higher-cost rolled-annealed (RA) copper foils. In the early stages of the process, the rigidflex design team members face a balancing act. They must define the mechanical challenges of the projected use cases balanced against the electrical performance requirements. These two considerations often butt heads, requiring the designer to balance and resolve the two. As a first step to arriving at the optimal design, the design team can gain considerable insight by producing a physical "paper-doll" mock-up of the circuit. The mockup pinpoints potential form and fit problems early in the design process. As modern CAD tools progress, they include 3D modeling of rigid-flex designs. The most up-to-date add animation. However, developing a 3D computer model involves considerable design steps, so an initial paper mock-up still proves to be informative. Rigid-Flex Design Best Practices The term "rigid-flex" points to one of the most significant design details. Rigid-flex circuit designs involve multiple elements that, when combined, result in a high level of complexity. For designers who develop rigid-flex designs, the biggest challenge remains: "How do I define all of the areas, layers, and stacks?" The answer: Use a table to define the stack layer design. As a general characteristic, most rigid-flex designs exhibit different layer stacks in different areas of the design. One simple way: Copy the design outline on a mechanical layer. Then create a fill-pattern that to identify the rigid and flexible portions of the design that contain a different layer stack as shown in Figure 2. The simplified design in Figure 2 uses the matching graphic fill patterns (the two columns on the right of the table) to identify the flexible and rigid areas of the board, respectively. For example, the layer named "Dielectric 1" is an FR-4 core. With the different layer stacks defined, any rigid-flex design team now confronts January 2014 • The PCB Design Magazine 29